Thermally-transferable polyester image-protecting layer

ABSTRACT

A thermal transfer medium for use in mass transfer printing comprises a substrate bearing on at least part of one surface thereof a coating of an overlay material comprising polyester having a glass transition greater than 50° C. (preferably at least 75° C.) and a molecular weight in the range 6,000 to 10,000. The overlay material combines good transfer characteristics, barrier properties and durability and is highly transparent.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national phase application based onPCT/GB00/02794, filed Jul. 20, 2000, and which further claims priorityfrom British Application No. 9919159.5, filed Aug. 14, 1999. Theseapplications in their entirety are incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to thermal transfer printing and concerns athermal transfer medium, a method of making the medium, a method offorming an overlay on a receiver material and the resulting receivermaterial bearing an overlay.

BACKGROUND TO THE INVENTION

Dye diffusion thermal transfer printing is a well known process in whichone or more thermally transferable dyes are transferred from selectedareas of a dyesheet to a receiver material by localised application ofheat, thereby to form an image. Full colour images can be produced inthis way using dyes of the three primary colours, yellow, magenta andcyan. Mass transfer printing is another well known technique in whichcolorant material (commonly carbon black) is transferred from a masstransfer medium to a receiver material by localised application of heat.Mass transfer printing is generally used to print monochrome images,commonly text, bar codes etc. Dye diffusion thermal transfer printingand mass transfer printing are often used in conjunction with oneanother, with a common application being the printing of personalisedcards such as identification cards, credit cards, driving licences etc,bearing a full colour image of the head of a person and text and/or abar code in monochrome (usually black). Such printing is convenientlycarried out using a dye sheet in the form of an elongate strip or ribbonof a heat-resistant substrate, typically polyethylene terephthalatefilm, carrying a plurality of similar sets of different coloured dyecoats and colorant, each set comprising a panel of each dye colour(yellow, magenta and cyan) and a panel of colorant, with the panelsbeing in the form of discrete stripes extending transverse to the lengthof the ribbon, and arranged in a repeated sequence along the length ofthe ribbon.

The resulting prints, particularly those in the form of cards, arefrequently carried in plastic pouches, but plasticisers in the pouches,are a particular problem because they are generally good solvents forthermal transfer dyes. A heavily plasticised PVC pouch, for example, canextract virtually all the colour from an unprotected image. As a resultit has become common practice to provide a layer of protective overlaymaterial over prints produced in this way. The overlay makes the printedcard or other material more secure by giving the image some degree ofprotection against abrasion and attack by plasticisers.

Overlay material is conveniently applied by thermal mass transfer, andto this end a ribbon-like dye sheet as described above conveniently alsoincludes a panel of mass transfer overlay material in each set,downstream of the dye panels and colorant panel.

For overlay material to perform satisfactorily the material should haveboth good printability and good protective properties. For goodprintability the material should have good transfer characteristics,which require the material to fracture easily during the printingprocess, giving clean edges and a continuous coating of the printedoverlay material. If the material does not fracture easily duringprinting the material instead tends to tear or rupture, producing imageswith jagged or ragged edges, exhibiting a phenomenon known as flashing.For good protective properties, the overlay should be flexible anddurable and capable of withstanding rough treatment and hostileenvironments, such as elevated temperatures, particularly when carriedin plastic pouches. To impart these properties, the overlay materialneeds to be tough and remain effectively continuous during prolongeduse.

The requirements for good printability and good protective propertiesare difficult to reconcile in a single material.

Current commercially available overlay material achieves the transfercharacteristics and durability requirements of the protective overlay bythree different main routes. One method provides a thin layer ofprotective overlay (<1 μm quoted, but more commonly about 0.2 μm) of avery strong durable polymer, containing a high loading of a smallparticulate filler (U.S. Pat. No. 5,387,573). A second method uses amulti-layer overlay comprising of a layer to aid release from thedyesheet substrate; a brittle, tough, durable polymer layer which haslow adhesion to the receiver material; and an adhesion promoting layerto allow the protective layer to adhere to the receiver material (U.S.Pat. No. 4,977,136). Another method uses a thick polymer layer of a verytough, durable polymer material which would normally have anunacceptable level of flashing, with a very high loading of anultra-violet light absorbing (UVA) filler material to achieve alightfast overlay, with a low cohesive strength to allow good transfer(WO 98/07578).

SUMMARY OF THE INVENTION

According to the present invention there is provided a thermal transfermedium comprising a substrate bearing on at least part of one surfacethereof a coating layer of a thermally transferable overlay material fortransfer onto a thermal transfer image formed on a receiver material,wherein the coating layer comprises polyester having a Tg greater than50° C. and a molecular weight in the range 6,000 to 10,000.

The polyester preferably has a Tg of at least 75° C.

Suitable commercially available polyesters include Skybon ES600-H(Skybon is a Trade Mark) from S K Chemicals, which has a Tg of about 80°C. and a molecular weight of about 7,000, and Vylon GXW27 (Vylon is aTrade Mark) from Toyobo, which has a Tg of about 77° C. and a molecularweight of about 7,500. Both of these materials are hydroxyl-terminatedpolyester resins. The polyester thus conveniently comprises ahydroxy-terminated polyester resin. In one embodiment the polyester hasa Tg of about 80° C. and a molecular weight of about 7,000 and inanother the polyester has a Tg of about 77° C. and a molecular weight ofabout 7,500. In these embodiments the polyester may be ahydroxy-terminated polyester resin.

Mixtures of suitable polyesters may be used.

By using a polyester having Tg and molecular weight characteristics asspecified, it is surprisingly found that an overlay material in the formof a single layer of material (in contrast to multi-layer overlays ofthe prior art) which is highly transparent and has good transfercharacteristics coupled with good barrier properties and durability canbe provided.

The coating suitably has a thickness in the range 0.5 to 5.0 μm,preferably 1.5 to 3.5 μm, typically 1.6 to 2.0 μm.

Various additives may optionally be included in the coating, eg toenhance or add properties in known manner.

For example, filler materials such as inorganic filler eg silica (SiO₂),alumina (Al₂O₃) and titanium dioxide (TiO₂) can be used to lower thecohesive strength of the polymer layer to aid transfer, but also toimprove durability and prevent ‘blocking’ (ie sticking) of the printedoverlay to other materials such as card wallets. Optical brighteners(OB) eg Uvitex OB (from Ciba Geigy) (Uvitex is a Trade Mark) may be usedto improve the colour of printed cards, as a tamper-proof measure in theoverlay, and to aid registration in the film coating process.Ultra-violet light absorbers (UVA) eg Tinuvin (from Ciba Geigy) (Tinuvinis a Trade Mark) can be used to give protection to both the overlay toreduce yellowing, and an underlying dye diffusion print to reduce fadingupon exposure to ultra-violet (UV) light.

The substrate may be any suitable heat-resistant material such as thoseknown in the art. Suitable substrate materials include films ofpolyesters, polyamides, polyimides, polycarbonates, polysulphones,polypropylene and cellophane. Biaxially oriented polyester film,particularly polyethylene terephthalate (PET), is currently favoured forits properties of mechanical strength, dimensional stability and heatresistance. The substrate suitably has a thickness in the range 1 to 20μm, preferably 2 to 10 μm, typically about 6 μm.

The thermal transfer medium preferably includes a subcoat between thesubstrate and coating, particularly in the form of a releasing subcoatto assist release of the coating during printing. One preferred releasesubcoat comprises a crosslinked acrylic coating.

The thermal transfer medium desirably includes a heat-resistantbackcoat, on the side of the substrate not carrying the coating, toresist applied heat in use in known manner.

The thermal transfer medium is conveniently in the form of a ribbon foruse in thermal transfer printing, comprising a substrate having on onesurface thereof a plurality of repeated sequences of dye coats and masstransfer materials in the form of discrete stripes extending transverseto the length of the ribbon.

Thus in a preferred aspect the invention provides a thermal transfermedium, comprising an elongate strip of substrate materials having onone surface thereof a plurality of similar sets of thermallytransferable dye coats and mass transfer layers, each set comprising arespective coat of each dye colour, yellow, magenta and cyan, and arespective mass transfer layer for colorant and overlay, each coat orlayer being in the form of a discrete stripe extending transverse to thelength of the substrate, with the sets arranged in a repeated sequencealong the length of the substrate, wherein each overlay material masstransfer layer comprises a coating of an overlay material comprisingpolyester having a glass transition temperature (Tg) greater than 50° C.and a molecular weight in the range 6,000 to 10,000.

The thermal transfer medium is conveniently made by dissolving ordispersing the overlay material in a suitable solvent as is well knownin the art to give a coating liquid. Suitable solvents include methylethyl ketone (MEK), propanone, tetrahydrofuran, toluene, cyclohexanoneetc. The coating liquid is then coated on the substrate and dried inknown manner eg by bar coating, blade coating, air knife coating,gravure coating, roll coating, screen coating, fountain coating, rodcoating, slide coating, curtain coating, doctor coating.

In a further aspect the invention provides a method of making thermaltransfer medium, comprising forming on one surface of a substrate acoating of an overlay material comprising polyester having a glasstransition temperature (Tg) greater than 50° C. and a molecular weightin the range 6,000 to 10,000.

The thermal transfer medium is used in known manner for forming anoverlay on a receiver material, frequently coupled with printing animage on suitable receiver material. The receiver material is typicallyin the form of a sheet or card of paper, cardboard, plastics materialetc having a suitable image-receiving surface. The thermal transfermedium is placed in contact with the receiver material and localisedheating effected to cause localised transfer of overlay material to forma protective overlay, commonly preceded by thermal transfer printing ofdyes to produce a full colour image and mass transfer of colorant toproduce text, a barcode etc, on the receiver material. One common use ofthe thermal transfer medium is in production of identification cards,typically formed on a sheet of plastics material such as polyvinylchloride, ABS and polyester, and which may bear a full colour photographof the head of an individual, produced by thermal transfer printing, incombination with text and/or a bar code produced by mass transferprinting of colorant, and covered with a layer of overlay material.

The invention finds particular application for use with receivermaterial in the form of a card of PVC with an image-receiving surfacecomprising vinyl chloride/vinyl acetate copolymer, and also withsynthetic laminated paper receivers and voided polyester receivers.

In a further aspect the invention provides a method of forming anoverlay on a receiver material, comprising superposing a thermaltransfer medium in accordance with the invention and a receivermaterial; and applying localised heating to the thermal transfer mediumto form an overlay on the receiver material.

The invention also includes within its scope the receiver materialbearing an overlay produced in this way, particularly an identificationcard bearing a full colour image produced by thermal transfer printingand text and/or a bar code produced by mass transfer printing ofcolorant.

The receiver material may optionally carry a further protective overlay(of similar or different constitution to the main overlay) on theopposed face.

The invention will be further described, by way of illustration, in thefollowing examples.

EXAMPLE 1 Comparative

A coating solution (solution A) was prepared from

-   -   Vylon GK640 30% by weight (TG=79° C./MWt.=20,000)    -   MEK 70% by weight

A coating was applied by hand using a Meier bar to give a wet coat about6 μm thick, onto a 6 μm thick polyester substrate base film. The basefilm was already coated with a heat resistant backcoat to provideprotection from a thermal head during the printing process, and subcoatcomprising a cross-linked acrylic system subcoat to provide release ofthe coating during printing. The coating was dried initially by a hairdrier, then in an oven at 110° C. for 30 seconds. The dry coat thicknesswas about 2.8 μm.

The subcoat comprises a highly cross-linked acrylic coating in which thecross-linking is achieved by UW-curing using a combination ofphotoinitiators and synergists included in the subcoat composition,details of which are given below. The subcoat was coated on thepolyester to give a dry coat thickness of approximately 0.5 μm. Thesubcoat composition, expressed as % w/w, was as follows:

Chemical % Composition Manufacturer MIBK 47.02%  Alcohols LTD UvecrylE1354 41.88%  UCB Radcure S.A Diakon MG102 5.98% KDT/Distrupol Irgacure907 1.68% Ciba Geigy Plastics Uvecryl P101 1.67% UCB Radcure S.AQuantacure ITX 0.84% Lambson Fine Chemicals Quantacure EPD 0.84% LambsonFine Chemicals Cyan dye 0.08%

-   MIBK is methyl iso-butyl ketone. This is the solvent from which the    subcoat layer is deposited. The solvent is evaporated from the    coating before it is subjected to UV-curing. Uvecryl E1354 is a    hexafunctional aromatic urethane acrylate oligomer. (Uvecryl is a    Trade Mark.)-   Diakon MG102 is a high molecular weight grade of poly    methylmethacrylate. (Diakon is a Trade Mark.)-   Irgacure 907, Uvecryl P101, Quantacure ITX & Quantacure EPD catalyse    UV-curing of the Uvecryl E1354. (Irgacure, Uvecryl and Quantacure    are Trade Marks.)

The resulting coating was spliced into a ribbon of dyesheet and was usedto print onto a receiver comprising a card of polyvinyl chloride (PVC).The surface of the PVC card consists predominantly of a vinylchloride/vinyl acetate copolymer (approximately 95:5 ratio,respectively). Printing was carried out using a Fargo Pro card printer(Fargo Pro is a Trade Mark) (manufactured by FARGO ElectronicsIncorporated).

The protective overlay was assessed for print transfer quality whichshowed very severe flashing and incomplete coverage of the PVC card. Nocards were tested due to the unacceptable transfer characteristics.

EXAMPLE 2 Comparative

A coating solution (solution B) was prepared from

-   -   Vylon GK130 30% by weight (Tg=15° C./MWt.range=5,000–8,000)        manufactured by Toyobo    -   MEK 70% by weight

A coating was applied as described in Example 1.

When the material was cut to size for splicing into a ribbon of dyesheetthe samples all ‘blocked’ together into a clump of dyesheet, with eachpiece of dyesheet ‘welded’ to the piece above in the stack.

No cards were printed for testing.

EXAMPLE 3

A coating solution (solution C) was prepared from

-   -   Vylon GXW27 30% by weight (Tg=77° C./MWt.range=7,500)    -   MEK 70% by weight

A coating was applied and printed as described in Example 1. The drycoat thickness was about 2.9 μm. The protective overlay was assessed forprint transfer quality. The overlay has sharp clean edges and thecoating is continuous over the printed area of card.

Further cards were produced and tested for durability (Taber test andtumble test), lightfastness and wallet barrier resistance (1) andcompared to currently commercially available material. In all testsperformed the protective overlay is equivalent or better than currentlycommercially available material. Test details are given below.

EXAMPLE 4

A coating solution (solution D) was prepared from

-   -   Skybon ES600-H 30% by weight (Tg=80° C./MWt.range=7,000)    -   MEK 70% by weight

A coating was applied and printed as described in Example 1. The drycoat thickness was about 3.2 μm. The protective overlay was assessed forprint transfer quality. The overlay has sharp clean edges and thecoating is continuous over the printed area of card.

Further cards were produced and tested for durability (Taber test andtumble test), lightfastness and wallet barrier resistance (1) andcompared to currently commercially available material. In all testsperformed the protective overlay is equivalent or better than currentlycommercially available material, with the exception of lightfastnesswhere the polymer only overlay yellowed with exposure to UV light.Further testing with the inclusion of UVAs and OBs significantly reducedthe yellowing of the overlay when exposed to UV light.

EXAMPLE 5

A coating solution (solution C) was prepared from

-   -   Vylon GXW27 30% by weight (Tg=77° C./MWt.range=7,500)    -   MEK 70% by weight

A coating was applied as described in Example 1, spliced into a ribbonof dyesheet and printed onto a voided polyester receiver (CP15 OlmecSecure from ICI Imagedata—Olmec is a Trade Mark) using a CP 15 printer(manufactured by Mitsubishi). The dry coat thickness was about 2.9 μm.The coating was assessed for transfer quality, which appeared very good.Further prints were made and tested for wallet barrier resistance (2),dye bleed and security properties. Test details are given below. In alltests performed the protective overlay is equivalent or better thancurrently commercially available material, with the exception of the dyebleed test, where the protective overlay allowed slightly more dye bleedthan currently commercially available material (which allows slight dyebleed).

EXAMPLE 6

A coating solution (solution D) was prepared from

-   -   Skybon ES600-H 30% by weight (Tg=80° C./MWt,range=7,000)    -   MEK 70% by weight

A coating was applied as described in Example 1, spliced into a ribbonof dyesheet and printed onto a voided polyester receiver (CP15 OlmecSecure) using a CP15 printer (manufactured by Mitsubishi). The dry coatthickness was about 3.2 μm. The coating was assessed for transferquality, which appeared very good. Further prints were made and testedfor wallet barrier resistance (2), dye bleed and security properties. Inall tests performed the protective overlay is equivalent or better thancurrently commercially available material.

EXAMPLE 7

A coating solution (solution C) was prepared from

-   -   Vylon GXW27 30% by weight (Tg=77° C./MWt.range=7,500)    -   MEK 70% by weight

A coating was applied as described in Example 1, spliced into a ribbonof dyesheet and printed onto a laminated paper receiver (CP700 OlmecSecure from ICI Imagedata) using a CP700 printer (manufactured byMitsubishi) fitted with an HX EPROM. The dry coat thickness was about2.9 μm. The coating was assessed for transfer quality, which appearedvery good.

EXAMPLE 8

A coating solution (solution D) was prepared from

-   -   Skybon ES600-H 30% by weight (Tg=80° C./MWt.range=7,000)    -   MEK 70% by weight

A coating was applied as described in Example 1, spliced into a ribbonof dyesheet and printed onto a laminated paper receiver (CP700 OlmecSecure from ICI Imagedata) using a CP700 printer (manufactured byMitsubishi) fitted with an HX EPROM. The dry coat thickness was about3.2 μm. The coating was assessed for transfer quality, which appearedvery good.

EXAMPLE 9

A coating solution was prepared from

-   -   Vylon GXW27 25% by weight (Tg=77° C./MWt.range=7,500)    -   Tinuvin 326 (UV absorber-Ciba Geigy) 0.5% by weight    -   Uvitex OB (optical brightener-Ciba Geigy 0.13% by weight    -   MEK 74.37% by weight

A coating was applied and printed as described in Example 1. The drycoat thickness was about 3.1 m. The protective overlay was assessed forprint transfer quality. The overlay had sharp clean edges and thecoating was continuous over the printed area of the card.

Further cards were produced and tested for durability (Taber test andtumble test), lightfastness and wallet barrier resistance (1) andcompared to currently commercially available material. In all testsperformed the protective overlay was equivalent or better than currentlycommercially available material.

EXAMPLE 10

A coating solution was prepared from

-   -   Vylon GXW27 25% by weight (Tg=77° C./MWt.range=7,500)    -   Tinuvin 326 (UV absorber-Ciba Geigy) 0.5% by weight    -   Uvitex OB (optical brightener-Ciba Geigy) 0.13% by weight    -   Aerosil MOX80 (silica filler-Degussa) 1.25% by weight    -   MEK 73.12% by weight

A coating was applied and printed as described in Example 1. The drycoat thickness was about 3.1 μm. The protective overlay was assessed forprint transfer quality. The overlay had sharp clean edges and thecoating was continuous over the printed area of the card.

Further cards were produced and tested for durability (Taber test andtumble test), lightfastness and wallet barrier resistance (1) andcompared to currently commercially available material. In all testsperformed the protective overlay was equivalent or better than currentlycommercially available material.

EXAMPLE 11

A coating solution was prepared from.

-   -   Skybon ES600-H 25% by weight (Tg=80° C./MWt.range=7,000)    -   Tinuvin 326 *UV absorber-Ciba Geigy) 0.5% by weight    -   Uvitex OB (optical brightener-Ciba Geigy) 0.13% by weight    -   MEK 74.37% by weight

A coating was applied and printed as described in Example 1. The drycoat thickness was about 3.1 μm. The protective overlay was assessed forprint transfer quality. The overlay had sharp clean edges and thecoating was continuous over the printed area of the card.

Further cards were produced and tested for durability (Taber test andtumble test), lightfastness and wallet barrier resistance (1) andcompared to currently commercially available material. In all testsperformed the protective overlay was equivalent or better than currentlycommercially available material.

EXAMPLE 12

A coating solution was prepared from.

-   -   Skybon ES600-H 25% by weight (Tg=80° C./MWt.range=7,000)    -   Tinuvin 326 *UV absorber-Ciba Geigy) 0.5% by weight    -   Uvitex OB (optical brightener-Ciba Geigy) 0.13% by weight    -   Aerosil MOX80 (silica filler-Dugussa) 1.25% by weight    -   MEK 73.12% by weight

A coating was applied and printed as described in Example 1. The drycoat thickness was about 2.9 μm. The protective overlay was assessed forprint transfer quality. The overlay had sharp clean edges and thecoating was continuous over the printed area of the card.

Further cards were produced and tested for durability (Taber test andtumble test), lightfastness and wallet barrier resistance (1) andcompared to currently commercially available material. In all testsperformed the protective overlay was equivalent or better than currentlycommercially available material.

Test Methods

Durability (1)—Taber Abrasion

The object of this test is to simulate the everyday abrasive wear to theprotective overlay on the PVC card surface which may be expectthroughout the lifetime of the card.

After printing the card with a special optical density (OD) imagedesigned for test purposes, with protective overlay as described in theExamples, the card is notched along the centre of the low opticaldensity long edge of the card to allow the card to be mounted as one ofa pair of test cards on the turntable of the Taber 5130 Abrader (Taberis a Trade Mark) (manufactured by Teledyne Taber) which wears down thesurface of the card with two abrasive rubber wheels under a specificload, driven by the sample in opposite directions.

The other card of the test pair is printed with a currently commerciallyavailable protective overlay. The card pair is then abraded for 100cycles using CS-10F wheels, 1 kg extra weight and a 70% vacuum level.

The performance of the development protective overlay is then be gradedagainst the commercially available material.

Good samples will show no loss of image but the protective overlay willbe scuffed; poor samples will have worn completely through to the cardsurface.

Durability (2)—Tumble Test

The object of this test is to simulate everyday wear of a card,including handling, flexing, heat and humidity, and abrasion.

After printing with an optical density image (as used in the Taber test)and protective overlay (as described in the Examples), cards are flexed100 times along the length of the card (image extension) using a testingmachine referred to as an AutoFlexer machine. The AutoFlexer machinecomprises of a pair of jaws, one fixed the other free to move in aforwards/backwards motion. A motor drives the jaws with a movement of 12mm and a closed gap of 41 mm with the cards flexing in the shortdirection (the set up may by altered to flex the cards in the longdirection with a closed gap of 55 mm). The jaws can accommodate amaximum of 4 test cards. The cards are flexed at 0.5 Hz.

After applying Veriderm cream (a hand cream designed to simulate naturalfinger grease, manufactured by Upjohn) (Veriderm is a Trade Mark) to theimaged surface, the card is placed in a 45° C./85% RH (relativehumidity) oven for 24 hours. The cards are then placed around the insidesurface of a cylindrical container (with the image facing inwards)filled with a selection of nuts and bolts (to simulate pocket change,keys, etc.). The lid of the container is then sealed and the containeris tumbled on a set of rollers at a speed of about 20 rpm for two hours.The cards are then removed, wiped clean of any excess grease, and gradedaccording to the level of damage to the card surface, as compared tocurrently commercially available material.

Good samples will show no loss of image but the protective overlay maybe scuffed; poor samples will have worn completely through to the cardsurface.

Wallet Barrier (1)

After printing with an optical density image (as used in the Taber test)and protective overlay (as described in the Examples), cards are flexed100 times along the length of the card (image extension) using theAutoFlexer machine. The flexed region of the card is examined by opticalmicroscope and a print made of any damage visible. A piece of theinternal surface of a PVC card wallet (as commonly used to clip an idcard to clothing) is placed over the imaged surface of the card, whichis then placed under a 1.2 kg mass in a 50° C. oven for 72 hours.Unflexed cards are also tested to indicate whether any dye bleed is dueto insufficient barrier properties of the protective overlay or tofracturing of the protective overlay during flexing. The samples areremoved from the oven, separated and graded according to the extent ofdye bleed through the protective overlay onto the PVC wallet, ascompared to currently commercially available material.

Good samples will have no dye bleed on to the PVC wallet material; poorsamples will show dye bleed even on unflexed cards.

Lightfastness

After printing with an optical density image (as used in the Taber Test)and protective overlay (as described in the Examples), cards aremeasured using a MacBeth TR 1224 densitometer (manufactured by MacBethDivision of Kollmorgen Instruments Corporation) (MacBeth is a TradeMark). The samples are then placed in an Atlas Ci35 Fade-ometer(manufactured by Atlas Electric Devices Company) (Atlas and Fade-ometerare Trade Marks) for exposure to:—

-   -   1.5 w/m² measured at 420 nm    -   290 J/m² measured at 420 nm    -   50% RH

The cards are then re-measured and the percent optical density lossrecorded, and graded for % OD loss and a visual assessment as comparedto currently commercially available material.

Good samples will be visibly brighter and more vibrant, with a low % ODloss measured; poor samples will be visibly faded in conjunction with ahigh % OD loss measured

Wallet Barrier (2)

After printing with a suitable image and protective overlay, the printedsurface is placed in contact with plasticised PVC wallet material, undera 1.2 kg mass in a 45° C./85% RH oven for 15 days. The samples areremoved from the oven, separated and graded according to the extent ofdye bleed through the protective overlay onto the PVC wallet material,as compared to currently commercially available material. No dye shouldbe seen to bleed onto the PVC wallet material.

Dye Bleed

The overlay was designed to protect the dye diffusion image from lowmolecular weight, migratable materials, resident in lamination overlayadhesives. These materials, should they enter the receiver layer, wouldcause the dyes to move, fuzzing out the detail in the photographs. Thistest assesses the effectiveness of the overlay to protect the image fromthe adhesive migratables.

Two prints of four passport size portraits images with protectiveoverlay are made. An adhesive thermal indicator strip is placed on anunprinted area of the receiver, and HMSO approved laminate is placedover both the imaged area and thermal indicator strip.

Test laminates incorporating thermal indicator strips are made to verifythat the lamination conditions are with 99–104° C.; then test prints arelaminated.

The two prints are placed in an oven set to 80° C. for 96 hours (4days).

To pass the test there must be no visible degradation of the image afterageing.

Security Test

A single portrait image is cut from a print of four passport sizedportrait images with protective overlay, and the printed image issecured using double sided adhesive tape to a piece of paper card. HMSOapproved laminate is applied over the printed image and card, havingpreviously made test laminates incorporating thermal indicator strips toverify the lamination conditions of 99–104° C.

Once cool the laminate around the print is cut using a scalpel.

The laminate is peeled slowly back, by hand, away from the print through180°. To pass the test the damage to the imaged surface must be suchthat neither the print or laminate can be re-used.

Summary of Results

Grading System

-   2 much better than current commercially available material-   1 better than current commercially available material-   0 as current commercially available material-   −1 worse than current commercially available material-   −2 much worse than current commercially available material

Ex- am- Wallet Light Wallet ple Tum- Barrier fast- Barrier Dye RefTransfer Taber ble (1) ness (2) Bleed Security 1 −2  n/a n/a n/a n/a n/an/a n/a 2 n/a n/a n/a n/a n/a n/a n/a n/a 3 0 0 0 1 1 n/a n/a n/a 4 0 00 1 1 n/a n/a n/a 5 0 n/a n/a n/a n/a 0 −1  0 6 0 n/a n/a n/a n/a 0 1 07 0 n/a n/a n/a n/a n/a n/a n/a 8 0 n/a n/a n/a n/a n/a n/a n/a 9 0 0 01 1 n/a n/a n/a 10 0 0 0 1 1 n/a n/a n/a 11 0 0 0 1 1 n/a n/a n/a 12 0 00 1 1 n/a n/a n/a

1. A thermal transfer medium comprising a substrate bearing on at leastpart of one surface thereof a coating layer of a thermally transferableoverlay material for transfer onto a thermal transfer image formed on areceiver material, wherein the coating layer comprises ahydroxy-terminated polyester having a Tg of at least 75° C. and amolecular weight ranging from 6,000 to 10,000, and wherein across-linked acrylic subcoat is present between the substrate andcoating.
 2. A thermal transfer medium according to claim 1, wherein thepolyester has a Tg of about 80° C. and a molecular weight ranging fromabout 7,000 to 10,000.
 3. A thermal transfer medium according to claim1, wherein the polyester has a Tg of about 77° C. and a molecular weightranging from about 7,500 to 10,000.
 4. A thermal transfer mediumaccording to claim 1, wherein the coating further comprises fillermaterial.
 5. A thermal transfer medium according to claim 1, wherein thecoating further comprises one or more ultra-violet light absorbers.
 6. Athermal transfer medium according to claim 1, wherein the coatingfurther comprises one or more optical brighteners.
 7. A thermal transfermedium according to claim 1, wherein the substrate comprises a film ofheat-resistant material selected from polyesters, polyamides,polyimides, polycarbonates, polysulphones, polypropylene and cellophane.8. A thermal transfer medium according to claim 1, wherein the coatinghas a thickness ranging from 0.5 to 5.0 μm.
 9. A thermal transfer mediumaccording to claim 1, wherein the other surface of the substrate has aheat-resistant backcoat.
 10. A thermal transfer medium, comprising anelongate strip of substrate material having on one surface thereof aplurality of similar sets of thermally transferable dye coats and masstransfer layers, each set comprising a respective coat of dye colors,yellow, magenta and cyan, and a respective mass transfer layer forcolorant and overlay, each coat or layer being in the form of a discretestripe extending transverse to the length of the substrate, wherein eachoverlay material mass transfer layer comprises a coating of an overlaymaterial comprising a hydroxy-terminated polyester having a glasstransition temperature (Tg) greater than 50° C. and a molecular weightranging from 6,000 to 10,000, and wherein a cross-linked acrylic subcoatis present between the substrate and coating.
 11. A method of forming anoverlay on a receiver material, comprising the steps of superposing athermal transfer medium in accordance with claim 1 and a receivermaterial; applying localized heating to the thermal transfer medium toform an overlay on the receiver material.
 12. A method according toclaim 11, further comprising the step of producing a printed image onthe receiver material by thermal transfer printing prior to formation ofthe overlay.
 13. A method according to claim 11, wherein the receivermaterial comprises a card of polyvinyl chloride.
 14. A method accordingto claim 11, wherein the receiver material has an image-receivingsurface comprising vinyl chloride/vinyl acetate copolymer.
 15. A methodaccording to claim 11, wherein the receiver is in the form of anidentification card bearing a full color image produced by thermaltransfer printing and text and/or a bar code produced by mass transferprinting of colorant.
 16. The combination of a receiver material havingan image-receiving surface comprising vinyl chloride/inyl acetatecopolymer and a thermal transfer medium comprising a substrate bearingon at least part of one surface thereof a coating layer of a thermallytransferable overlay material for transfer onto a thermal transfer imageformed on the receiver material, wherein the coating layer comprises ahydroxy-terminated polyester having a Tg greater than 50° C. and amolecular weight ranging from 6,000 to 10,000, and wherein across-linked acrylic subcoat is present between the substrate andcoating.
 17. A combination according to claim 16, wherein the receivermaterial comprises a card of polyvinyl chloride.
 18. A method of formingan overlay on a receiver material having an image-receiving surfacecomprising vinyl chloride/vinyl acetate copolymer, comprising the stepsof superimposing a thermal transfer medium comprising a substratebearing on at least part of one surface thereof a coating layer of athermally transferable overlay material for transfer onto a thermaltransfer image formed on a receiver material, wherein the coating layercomprises a hydroxy-terminated polyester having a Tg greater than 50° C.and a molecular weight ranging from 6,000 to 10,000, and wherein across-linked acrylic subcoat is present between the substrate andcoating; and applying localized heating to the thermal transfer mediumto form an overlay on the receiver material.